Audio encoders are well known. These encoders are operable to receive one or more input audio signals and process them to generate corresponding bit-streams of encoded output data. Such processing executed within the audio encoders involves partitioning the one or more input signals into segments, and then processing each segment to generate its corresponding portion of data for inclusion in the encoding output data.
Conventional methods of creating such bit-streams employ fixed uniform time segments. Beneficially, the segments are at least partially overlapping. An example of an encoder performing in this manner is Philips Electronics N.V.'s proprietary SSC codec whose mode of operation is now included in a known international standard MPEG 4 extension 2, namely text of ISO/IEC 14496-3:2002/PDAM 2 concerning “Parametric coding for High Quality Audio”.
Other methods of encoding audio signals have been proposed. For example, in a published international PCT application no. PCT/SE00/01887 (WO 01/26095), there are described modern audio encoders which employ adaptive window switching, namely the audio encoders switch time segment lengths depending on input signal statistics. In one implementation, non-uniform time and frequency sampling of a spectral envelope of an input signal is achieved by adaptively grouping sub-band samples from a fixed size filter-bank into frequency bands and time segments, each of which generates one envelope sample. This allows for instantaneous selection of arbitrary time and frequency resolution within the limits of the filter-bank. Such encoders preferably default to relatively long time segments and a fine frequency resolution. In the temporal vicinity of signal transients, relatively shorter time segments are used, whereby larger frequency steps can be employed in order to keep data size within limits. Moreover, to enhance benefits from such non-uniform temporal sampling, variable length of bit-stream frames are used.